Just because Pakistan is assembling the missiles, doesn't mean that the technology is developed in Pakistan. Pakistan hasn't added a single kilometer to the range of the missiles since a decade.
Also, if Pakistan had the basic rocket technology developed for the missiles, it would have been easy to develop it further for SLVs. There is big difference between developing the technology from scratch and just getting it from others. When the technology is developed, there is a complete know-how on the technology and can be further developed and improved.